Sorry to hear your baby is not well and in pain during her bath. I would advise you to see a dermatologist as soon as possible, so that the rash can be looked at properly and appropriate creams prescribed.
It is not common for babies to get fungal infections of their buttocks. It is more likely this is due to irritant contact dermatitis where the skin is irritated by frequent contact with her urine and faeces.
You can try moisturisers and regular nappy changes as a start but it is most likely she will regular a short course of treatment with a mild steroid cream to improve the rash. Hope she feels better very soon!
Dr Stephanie Ho, Dermatologist